In developing a flowsheet
for the production of a chemical, it is desirable to consider the
environmental ramifications of using each unit operation in the process
rather than postponing this consideration until the flowsheet is finished.
This "front end" environmental assessment is more likely
to result in a chemical process that has less potential to cause environmental
harm. In many instances, this environmentally benign design will also
be more profitable, the improved design will require lower waste treatment
and environmental compliance costs and will convert a higher percentage
of raw materials into salable product.

In considering pollution prevention for unit
operations in the design of chemical processes, the following considerations
are important.

Material Selection: Many of the
environmental concerns can be addressed by reviewing material
properties and making the correct choice of unit operation
and operating conditions. The materials used in each unit
operation should be carefully considered so as to minimize
the human health impact and environmental damage of any
releases that might occur.

Operating Conditions: The operating
conditions of each unit should be optimized in order to
achieve maximum reactor conversion and separation efficiencies.

Material Storage and Transfer: The
best material storage and transfer technologies should be
considered in order to minimize releases of materials to
the environment.

Energy Consumption: Energy consumption
in each unit should be carefully reviewed so as to reasonable
minimize its use and the associated release of utility-related
emissions.

Process Safety: The safety ramifications
of pollution prevention measures need to be reviewed in
order to maintain safe working conditions.

In the following sections, we apply this
framework for preventing pollution in unit operations by considering
choices in materials, technology selection, energy consumption,
and safety ramifications. In Section 9.2, material choices that
are generic to most chemical processes, like process water and fuel
type, are analyzed with respect to in-process waste generation and
emission release. Other process materials that are more specific
to various unit operations are discussed in subsequent sections
of this chapter. Chemical reactors are the topic in Section 9.3.The
environmental issues related to the use of reactants, diluents,
solvents, and catalysts are discussed first. Then, the effects of
reaction type and order on product yield and selectivity are covered.
The effects of reaction conditions (temperature and mixing intensity)
on selectivity and yield are illustrated. Finally, the benefits
of additional reactor modifications for pollution prevention are
tabulated. In Section 9.4, the most important topics include the
choice of material (mass separating agent) to be used in separations,
design heuristics, and examples of the use of separation technologies
for recovery of valuable components from waste streams, leading
eventually to their reuse in the process. Separative reactors are
the topic of discussion in Section 9.5. These hybrid unit operations
have special characteristics to help achieve higher conversions
and yields in chemical reactors compared to conventional reactor
configurations. In Section 9.6, methods for reducing emissions from
storage tanks and fugitive sources are discussed. The safety aspects
of pollution prevention and unit operations are the topic of Section
9.7.It will be shown that many pollution prevention efforts tend
to make chemical processes more complex, necessitating a higher
level of safety awareness.

In making pollution prevention decisions
that include choices of materials, unit operations technologies,
operating conditions, and energy consumption, it is very important
to consider health and environmental risk factors. It is also of
high importance to incorporate cost factors and safety ramifications.
In Section 9.8, review a method for evaluating health risk into
unit operations decisions by considering the optimum reactor operating
conditions as an example application. Although no generally accepted
method exists for these risk assessments, the method outlined in
Chapter 8 and applied in Section 9.8 is useful for incorporating
multiple risk factors into decisions regarding item operations.

Finally, it is also important to introduce
the concept of "risk shifting." Pollution prevention decisions
that are targeted to reduce one kind of risk may increase the level
of risk in other areas. For example, a common method for conserving
water resources at chemical manufacturing facilities is to employ
cooling towers. Process water used for cooling purposes can be recycled
and reused many times. However, there is an increased risk to workers
who may be exposed to the biocides used to control microbial growth
in the cooling water circuit. Also, in some cooling water processes,
hazardous waste is created by the accumulation of solids-for example,
from the use of hexavalent chromium (a cancer causing agent) as
a corrosion inhibitor.

Another example of shifting risk from the
environment and the general population to workers involves fugitive
sources (valves, pumps, pipe connectors, etc.).One strategy for
reducing fugitive emissions is to reduce the number of these units
by eliminating backup units and redundancy. This strategy will reduce
routine air releases but will increase the probability of a catastrophic
release or other safety incidents. Simply put, the objective of
pollution prevention is to reduce the overall level of risk in all
areas and not to shift risk from one type to another.

Chapter 9. Example Problem

Example problem 9.3-3

Estimate the magnitude of the mixing effect
on reaction yield.

A second-order competitive-consecutive reaction
is being carried out in a CSTR. The initial concentration of reactant
A in the vessel is 0.2 gmole/liter and the feed containing reactant
B is introduced into the reactor at the impeller. The volume of
the vessel is 100 liters, the impeller diameter is 0.5 ft, k1 is
35 liter / (gmolesec), impeller speed is 200 rpm. Additional
data is shown below. Estimate the reaction yield as a fraction of
the expected yield.

Additional Data:

Lf = 0.5 ft (impeller diameter
for feed at impeller tip)

n = kinematic viscosity of mixture = 1.08
cs = 1.16x10-5ft2/sec

Solution

The x-axis in Figure 9-3.6 requires that t be calculated, and t
requires u.

From Figure 9-3.7, the estimated value of
Y/ Yexp is approximately 0.94. Thus, the mixing in this
reactor is almost sufficient to achieve the expected yield. Byproduct
generation is not affected to a large degree by mixing in this CSTR,
but could be improved slightly by operating the mixer at higher
speeds.

Chapter 9. Sample Homework Problem

Solvent Choice for Caffeine Extraction
from Coffee Beans

About 20% of the coffee consumed in the United
States is decaffinated. There are many solvents and processes developed
over the past century to accomplish this step. Critical issues in
choosing a solvent are the caffeine/solvent affinity, the cost of
the solvent, the ease of caffeine recovery from the solvent, safety
aspects, and environmental impacts. The original process used a
synthetic organic solvent to extract caffeine from un-roasted coffee
beans. These solvents included trichloroethylene (C2HCl3) and methylene
chloride (CH2Cl2). Today, caffeine is extracted using natural
solvents including supercritical carbon dioxide, ethyl acetate (naturally
found in coffee), oils extracted from roasted coffee, and water.
Using Material Safety Data Sheets as a source of information, rank
order these solvent candidates based only on their toxicological
properties. Do not consider the extracted oils since
the identity of these is not available. Use PEL and/or LD50 (rat)
toxicological data.